Electromechanical oscillation is an inherent property of an AC transmission system and cannot be entirely eliminated. In many cases, damping of inter-area electromechanical oscillation sets limits for power transfer capacity. In addition, the oscillations may pose a serious threat to system security if they are not controlled properly. From an operational point of view, it is of high priority to be able to estimate the damping of the oscillations reliably in real-time in order to take appropriate and timely measures to keep the system stable.
ABB has a first generation of the electromechanical oscillation detection software implemented in a system named PSGuard. In PSGuard there is a function Power Oscillation Monitoring (POM) used for the detection of power swings in a high voltage power system. The POM function processes a single input signal from Phasor Measurement Units (PMU) and detects various power swing modes, which can be used to detect disturbances.
POM can quickly detect activity of swing modes that are observable in a measurement from for example, a Wide-Area Measurement System (WAMS). Additionally POM indicates the frequency of an oscillation, which may then be compared with existing known modes of the power system e.g., the operator may distinguish if a local or inter-area mode is excited.
The PSGuard system nowadays also has a further function, Power Damping Monitoring (PDM), which uses a similar approach as the POM with the difference that multiple measurements from different locations can be used simultaneously and in general longer time windows of data. PDM is a complement to POM in the sense that the POM can faster detect oscillations arising from disturbances or switching events in the power system, whereas the PDM can better estimate the damping level during normal operating conditions.
PDM is based on the applying of measurement values from a number of PMUs in a model of the system in order to determine system resonances.
One type of function similar to PDM is a so-called Prony analysis, which is described in U.S. Pat. No. 7,987,059. Here measurements are applied in a first and a second state space model in order to obtain resonance frequencies of the system, where the second model is a modally transformed variation of the first state space model.
The use of Prony analysis is also discussed by Jaime Quintero et al in “An Oscillation Monitoring System for Real-time Detection of Small-Signal Instability in Large Electric Power Systems”, 2007 IEEE Power Engineering Society General Meeting, Jun. 24-28 2007, Tampa, Fla., USA.
It is thus possible to find out what resonance frequencies there are in the system. However, there is room for improvement in the field of power oscillations damping, especially concerning how serious these resonances are in relation to system stability.
It would therefore be desirable for an improvement on this situation.